Sodicity v. yield decline functions for a Vertisol (Grey Vertosol) under border check and raised bed irrigation

Abstract

The grain yields of 2 soybean crops and a canola crop were related to topsoil and subsoil properties that are known to affect soil aggregate stability in water. A broad range of soil conditions was produced by applying gypsum and lime at various rates to a Vertisol that was naturally sodic throughout the soil profile. The observations were made under both ‘border check’ and ‘raised bed’ irrigation.

Soil factors that influence dispersibility and crop growth were considered as part of a stepwise regression analysis. For the 2 soybean crops under border check irrigation, surface exchangeable sodium percentage (ESP) explained 72 and 63%, respectively, of the grain yield variation. For the canola crop that followed, a combination of pH, ESP and electrical conductivity (in that order) explained 75% of the variation in grain yield.

Grain yields for the 3 crops declined as the soil became more sodic, particularly on the flat border check area. For soybeans (flat area), the critical values of ESP at which surface sodicity caused a 50% yield decline (ESP₅₀) were strongly influenced by the post-planting climatic conditions. When there was heavy rain just after germination of the soybean seedlings, the ESP₅₀ value was 8.6. The more sodic areas developed a thick crust that impeded seedling emergence. However, in the following year, when conditions remained dry after planting, the ESP₅₀ value was 10.7.

Under bed irrigation, the influence of ESP on yield was only minor after the first year of cropping. There was evidence of soil structure improvement from salt that was deposited in the beds via the irrigation water. Subsoil chemical properties that affect structural stability had more of an influence on crop yield under beds than under the flat areas.